The present invention relates to a coder-decoder for digital transmission channels and is intended more particularly for use in digital telephony.
The presently used pulse-code modulators and which are also standardized by the CCITT (TNE1 with publications G711 and G732) code the telephone signal level with 8 times on 8 bits so as to permit a large number of successive codings and decodings without making the quantization noise disturbing.
Unfortunately, in the presence of high additive noise (case of long radio links and artificial satellite links) the error rate on the bits becomes high and acts in the same way on all the bits, no matter what their weight. This has a particularly prejudicial effect on the high weight bits and particularly sign bits.
Thus, a bit error rate of 10.sup.-3 corresponds to the acceptable threshold for a telephone channel, whilst a bit error rate of 10.sup.-1 would be considered satisfactory with a modulator where all the bits have the same weight (e.g. a "DELTA"-type modulator). These effects are also serious when it is a question of transmitting a data-modulated analog signal.
In connection with these problems, reference can be made to the article by C. BERGER entitled "Choice for digital transmissions: pulse-code modulation or delta modulation" and published in the Journal "L'Onde Electrique", Vol. 59, No. 5, pp. 55 to 62, May 1979.
One solution for this problem consists of coding the complete digital train on transmission, whilst adding redundant bits. However, this solution has a number of disadvantages. Thus, it significantly increases the line throughput, which increases the spectral occupation and no longer permits higher order time multiplexing by hierarchic multiplexers. It makes the decoder operate at high speeds and it only has about 500 nanoseconds to take its decision in the case of a 2.048 Mbit/s system, which causes constructional problems and increases costs. It calls for the use of transmission systems which do not lead to the appearance of error groups, so that the decoder is made inefficient.